Make BridgeDB report internal metrics

We're done with #9316 (moved), which means that we have code in place that allows BridgeDB to export metrics. So far, all metrics are user-centric, meaning that they focus on how BridgeDB users interact with the system. BridgeDB-centric metrics would help us debug and understand BridgeDB. The following come to mind:

• Number of bridges per distribution ring.
• Number of bridges per transport, similar to assignments.log (originally proposed in #14453 (moved))
• Number of requests for which we had no bridges.

We could also incorporate bridge assignments in our metrics, so we don't have to report them separately in the assignments.log file (see #29480 (moved)). Let's not forget to update BridgeDB's metrics specification.